Fluorescence

The Technical

Diamond fluorescence is a form of illumination that is created when a diamond is exposed to Ultraviolet (UV) light. About a third of all diamonds will glow under ultraviolet light, usually a blue colour which may be noticeable under a black light or strong sunlight. According to the GIA, who reviewed a random sample of 26,010 natural diamonds, 65% of the sample had no fluorescence. Of the 35% that did have fluorescence, 97% had blue fluorescence of which 38% had faint blue fluorescence and 62% had fluorescence that ranged from medium to very strong blue. Other colours that diamonds can fluoresce are green, yellow, white and red but are very rare and are sometimes a combination of the colours such as blue-green or orange.

The Real

Some diamonds with “very strong” fluorescence can have a “milky” or “oily” look to them, but they are also very rare and are termed “over-blues.” Their study concluded that with the exception of “over-blues” and yellow fluorescent diamonds, fluorescence had little effect on transparency and that the strong and very strong blue fluorescent stones on average had better colour appearance than non-fluorescents. Since blue is a complimentary colour to yellow and can appear to cancel it out, strong blue fluorescence had especially better colour appearance with lower colour graded diamonds that have a slight yellowish tint such as “I” colour or “J” colour but had little effect on the more colourless “D” through “F” colour grades.

The GIA released the results of a study in 1997 that addressed the effect of blue fluorescence on the appearance of diamonds. Here are some of the conclusions of that study:

In general, the level of fluorescence had no widely perceptible effect on the colour appearance of diamonds viewed table down (upside down).

In the table up position, diamonds described as Strong or Very Strong fluorescence appeared to have better colour than less fluorescent stones.

The effect of fluorescence on colour was most noticeable in the lower (I-J-K) colours.

Non-trade observers in the test had more difficulty than those in the diamond trade in seeing meaningful distinctions of fluorescence on colour appearance or transparency.

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